---
layout: docs
page_title: Sentinel policy reference
description: >-
  Use the Nomad Enterprise Sentinel policy feature to implement fine-grained control over jobs, such as restricting when jobs can run or ensuring jobs use specific images. Learn about policy structure, job objects, Access Control List (ACL) token objects, and namespace objects.
---

# Sentinel Policy Reference

<EnterpriseAlert product="nomad" />

This page provides reference information on using the Nomad Sentinel policy feature to implement fine-grained policies such as restricting when jobs can run or ensuring job use specific images. Learn about policy structure, job objects, Access Control List (ACL) token objects, and namespace objects.

## Introduction to Sentinel Policies

In Nomad Enterprise, operators can create Sentinel policies for fine-grained
policy enforcement. Sentinel policies build on top of the ACL system and allow
operators to define policies such as disallowing jobs to be submitted to
production on Fridays or only allowing users to run jobs that use pre-authorized
Docker images. Sentinel policies are defined as code, giving operators
considerable flexibility to meet compliance requirements.

Refer to the [Nomad Sentinel Tutorial][] for more information about deploying
Sentinel policies, as well as the documentation for the [`nomad sentinel`][]
subcommands.

## Sentinel Policy Structure

Sentinel policies are specified in the [Sentinel Language][sentinel]. The
language is designed to be understandable for people who are reading and writing
policies, while remaining fast to evaluate. There is no limitation on how
complex policies can be, but they are in the execution path so care should be
taken to avoid adversely impacting performance.

In each scope, there are different objects made available for introspection,
such a job being submitted. Policies can inspect these objects to apply
fine-grained policies. Nomad supports a `submit-job` scope for registering jobs,
and a `submit-host-volume` scope for creating or updating dynamic host volumes.

### submit-job scope

The following top-level objects are available to policies in the `submit-job`
scope automatically, without an explicit import.

* `job`: the submitted job. This is a [Sentinel Job
  Object](#sentinel-job-objects).
* `existing_job`: the previous version of the job. If `job_exists` is true, this
  is always non-nil. This is also a [Sentinel Job
  Object](#sentinel-job-objects).
* `job_exists`: a boolean field that indicates that a previous version of the
  job exists.
* `nomad_acl_token`: the ACL token the job was submitted with. This is a
  [Sentinel Nomad ACL Token Object](#sentinel-acl-token-objects).
* `namespace`: the namespace the job is in. This is a [Sentinel Nomad Namespace
  Object](#sentinel-namespace-objects).

### submit-host-volume scope

The following top-level objects are available to policies in the
`submit-host-volume` scope automatically, without an explicit import.

* `volume`: the submitted volume. This is a [Sentinel Dynamic Host Volume
  Object](#sentinel-dynamic-host-volume-objects).
* `existing_volume`: the previous version of the volume. If `volume_exists` is
  true, this is always non-nil. This is also a [Sentinel Dynamic Host
  Volume Object](#sentinel-dynamic-host-volume-objects). `volume_exists`: a
  boolean field that indicates that a previous version of the volume exists.
* `nomad_acl_token`: the ACL token the job was submitted with. This is a
  [Sentinel Nomad ACL Token Object](#sentinel-acl-token-objects).
* `namespace`: the namespace the job is in. This is a [Sentinel Nomad Namespace
  Object](#sentinel-namespace-objects).
* `node`: the node the volume has been placed on. This is a [Sentinel Node
  Object](#sentinel-node-objects).
* `node_pool`: the node pool of the node the volume has been placed on. This is
  a [Sentinel Node Pool Object](#sentinel-node-pool-objects).

Sentinel convention for identifiers is lower case and separated by underscores.
All fields on an object are accessed by the same name, converted to lower case
and separating camel case to underscores.

## Sentinel Job Objects

The `job` and `existing_job` objects map to the [JSON job specification][], with
the fields converted to the Sentinel convention. Here are some examples:

| Job Field                                | Sentinel Accessor                           |
|------------------------------------------|---------------------------------------------|
| `job.ID`                                 | `job.id`                                    |
| `job.AllAtOnce`                          | `job.all_at_once`                           |
| `job.ParentID`                           | `job.parent_id`                             |
| `job.TaskGroups`                         | `job.task_groups`                           |
| `job.TaskGroups[0].EphemeralDisk.SizeMB` | `job.task_groups[0].ephemeral_disk.size_mb` |

## Sentinel ACL Token Objects

The `nomad_acl_token` object maps to the [ACL token][], with the fields
converted to the Sentinel convention. Here are some examples:

| Nomad ACL Token Field         | Sentinel Accessor             |
|-------------------------------|-------------------------------|
| `nomad_acl_token.AccessorID`  | `nomad_acl_token.accessor_id` |
| `nomad_acl_token.Policies[0]` | `nomad_acl_token.policies[0]` |

Note that the `SecretID` field is always redacted to prevent credential leaks.

## Sentinel Namespace Objects

The `namespace` object maps to the [Namespace][], with the fields converted to
the Sentinel convention. Here are some examples:

| Namespace Field                              | Sentinel Accessor                              |
|----------------------------------------------|------------------------------------------------|
| `namespace.Description`                      | `namespace.description`                        |
| `namespace.NodePoolConfiguration.Allowed[0]` | `namespace.node_pool_configuration.allowed[0]` |

## Sentinel Node Objects

The `node` object maps to the [Node][], with the fields converted to the
Sentinel convention. Here are some examples:

| Node Field         | Sentinel Accessor  |
|--------------------|--------------------|
| `node.Class`       | `node.class`       |
| `node.Meta["foo"]` | `node.meta["foo"]` |

## Sentinel Node Pool Objects

The `node_pool` object maps to the [Node Pool][], with the fields converted to
the Sentinel convention. Here are some examples:

| Node Pool Field                                       | Sentinel Accessor                                       |
|-------------------------------------------------------|---------------------------------------------------------|
| `node_pool.Description`                               | `node_pool.description`                                 |
| `node_pool.SchedulerConfiguration.SchedulerAlgorithm` | `node_pool.scheduler_configuration.scheduler_algorithm` |

## Sentinel Dynamic Host Volume Objects

The `volume` object maps to the [Dynamic Host Volume][], with the fields
converted to the Sentinel convention. Here are some examples:

| Dynamic Host Volume Field                    | Sentinel Accessor                              |
|----------------------------------------------|------------------------------------------------|
| `volume.Name`                                | `volume.name`                                  |
| `volume.RequestedCapabilities[0].AccessMode` | `volume.requested_capabilities[0].access_mode` |


[Nomad Sentinel Tutorial]: /nomad/docs/govern/sentinel
[`nomad sentinel`]: /nomad/commands/sentinel
[sentinel]: https://docs.hashicorp.com/sentinel
[JSON job specification]: /nomad/api-docs/json-jobs
[ACL token]: https://github.com/hashicorp/nomad/blob/v1.9.4/nomad/structs/structs.go#L13502-L13531
[Namespace]: https://github.com/hashicorp/nomad/blob/v1.9.4/nomad/structs/structs.go#L5578-L5610
[Node]: https://github.com/hashicorp/nomad/blob/v1.9.4/nomad/structs/structs.go#L2086-L2210
[Node Pool]: https://github.com/hashicorp/nomad/blob/v1.9.4/nomad/structs/node_pool.go#L46-L68
[Dynamic Host Volume]: https://github.com/hashicorp/nomad/blob/main/nomad/structs/host_volumes.go#L18-L87
